Zone-refining apparatus



Dec. 11, 1956 c. G. SMITH Filed Jan. 26. 1953 l "m mw GENERATOR zoNE-'REFINING APPARATUS 2 Sheets-Sheet l /N VENTO/2 CHARLES GSM/TH 5V @Zw/a, A T RNEY Dec. 11, 1956 c. G. SMITH 2,773,923

ZONE-REFINING APPARATUS Filed Jan-v. 26. 1953 2 Sheets-Sheet 2 /NVE/vm/ CHARLES G. SM/TH United States Patent O ZONE-REFINING APPARATUS Charles G. Smith, Weston, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application January 26, 1953, Serial No. 333,219

6 Claims. (Cl. 13-26) This invention relates to a zone-refining apparatus to be used in the manufacture of transistors, and specifically to the structure used to hold the material to be refined.

It is well known that in the manufacture of transistors, the concentration of p-type and n-type impurities in the semiconductor crystal must be controlled within critical limits. To be certain that these limits are not exceeded, p

the semiconductor material used, germanium, for example, should be quite pure. The improved zone-rening apparatus described herein is excellent for such purifying processes, and the degree of purification obtained by its use is far greater than that realized in previous operations of a similar nature.

In zone-rening processes it has been determined that the width of the molten zones should be as small as possible because the degree of purification obtained increases exponentially with the inverse of the liquid zone widths of the material being refined. For example, if a graphite trough about a foot long is used to hold an impure bar of germanium, and if six high-frequency heating coils are properly spaced to create a series of molten zones in the bar, it is known that as the bar is moved through the zone-retining apparatus and the zones are formed, the impurities therein will be progressively swept toward the tail end of the bar. After the bar has been cooled, the

end containing the impurities may be cut off and the remainder is pure germanium. However, where an ordinary graphite trough is used to hold the material being purified, thermal conduction along the length of the trough will cause an increase in the width of the molten zones. In other words, the heat, instead of being led inward to form narrow molten zones of germanium, is spread along the length of the trough and widens these zones considerably.

This invention involves an improved zone-refining system which assures smaller liquid zone widths and, hence, yields a pronounced gain in purification. Thetrough is designed to bring it as close as possible to the high-frequency coils and the width of the heating coils is kept to a minimum. Both of these features improve the purity of the refined material. However, the feature of this invention which produces its major advantage resides in a plurality of transverse slots which are cut into the trough and are spaced along its length. These slots act to Vsubstantially decrease the flow of heat along the longitudinal axis of the trough. Thus, the heat is concentrated in narrow zones and puriiicaton is greatly increased over that previously obtained by using prior structures.

This invention and features thereof will be understood more clearly and fully from the following detailed description of one embodiment of the invention with reference to the accompanying drawings in which:

Fig. 1 is a schematic view of the zone-refining apparatus;

Fig. 2 is an isometric View of the zone-refining trough and the material to be refined;

Fig. 3 is a cross-sectional view of the zone-renng "ice v2 trough and the material to be refined as they are moved through three of the high-frequency heating coils;

Fig. 4 is a top view of the zone-refining trough;

Fig. 5 is a partial side View of the zone-refining trough; and

Fig. 6 is `a cross-section of the zone-refining trough taken along line 6-6 of Fig. 4.

Referring now to Fig. l, a zone-refining apparatus of `the type employed in the process of purifying semiconductor materials includes an elongated trough 1, containing the substance to be refined. The trough 1 may be moved along a quartz tube 2 having six spaced highfrequency heating coils 3, 4, 5, 6, 7, and 8 so that 1ocalized regions of the material can be progressively melted. The high-frequency coils are connected in series to a 450 kilocycle generator 9 which provides the power necessary to heat the trough 1 to the proper temperature. The trough 1 is pulled through the tube 2 by a quartz pull-rod 10 which is connected to the trough at one end and to a control 11 at the other end. The control 11 may be 'set so that the trough 1 is pulled through the tube 2 at a predetermined rate. The substance to be zone re- Vlined should be heated in an atmosphere of inert gas, hydrogen, or in a vacuum to prevent it from oxidizing or reacting with atmospheric impurities. Therefore, helium, for example, is passed into the tube 2 through an intake tube 12 and is removed through an exhaust tube '13 throughout the operation. Removable seals 14 and 15 close tube 2 and support the intake tube 12 and the exhaust tube 13. The seal 14 also acts as a guide for the pull-rod 10. Supports 16 and 17 hold the tube 2 at the correct angle. About a ive degree slope in the direction of pulling has been used successfully to obtain an even distribution of the substance being purified within th trough 1.

Thus, for example, if a graphite trough 12 inches longV progressively swept toward the tail end of the trough.

When the rening process is complete and the germanium is cool, the front two-thirds of the bar, more or less, are found to be pure. Therefore, the impure third may be removed and the remainder is ready for further processing.

Fig. 2 shows a trough 1 made in accordance with this invention. This trough may be, for example, an elongated Abody about 16 inches in length made of pure graphite, and having a recess about 1% inches deep there# in adapted to receive a semiconductor material 18, such as a bar of germanium. The trough 1 has transverse slots 19 spaced along its length. These slots 19 are cutideep enough to substantially decrease thermal conduction along the longitudinal axis of the trough 1, but are `not deep or wide enough to weaken the side wallsof the trough. In the case of the trough 1, as shown in Figsl' 2, 3, 4, 5, and 6, the slots 19 may be about 1A inch deep,

spaced about 1A inch apart, and approximately 1,{,2'inch in width. However, the slots 19 should be wide enough to allow the inert gas passing over the trough 1 and through the slots to convectively cool the solid zones 22 of the semiconductor material 18, as shown in Fig." 3.

Helium has been used with good results to obtain con'- vective cooling, and it also acts to prevent the semiconductor material 18 from oxidizing or reacting with atmoswhereby the trough can be pulled throughrthe tubefr2.

Thus they trough may be readily moved through a quartz/1` tube 2,: such as that shown in Fig. 1, having a diameter of about 2 inches. is such that it is brought as close as possible to the heating coils so that the electrical coupling between the high frequency heating coils and the trough is an optimum. The depth of the semiconductor material 18 in the trough 1 should be such that the level of the material lies below the level of the slots 19, as shown in Fig. 2, but should not be high enough to allow cooling to essentially affect the temperature of the open top of the semiconductor material 18 during the zone-refining process;

IIt should be noted that the ends -of the trough 1 are reduced so that the diameter of these ends -is ldecreased, for example, about .04 inch, as shown in Figs. 2, 4, and 5 by the reduced sections 23. There is a tendenecy, due to increased coupling which occurs between the more solid ends of the trough 1 and the heating coils, for these ends to become more highly heated than the intermediate sec tion of the trough. I-f these solid ends are not reduced, the quartz tube 2 tends to crystallize under repeated heating at these higher temperatures. The recessed sections 23 prevent the ends of the trough 1 yfrom touching the tube 2 so that the heat transferred from the recessed sections 23 to the quartz tube 2 is decreased substantially and the quartz tube 2 is protected from crystalline deterioration.

Referring now to Fig. 2 and Fig. 3, which shows the trough as it appears with the semiconductor lmateria-l during an intermediate stage of the zone-refining pro-cess, the trough 1 is pulled through the quartz tube 2 by the pull-rod and is heated by three of the high-frequency coils 5, 6, and 7. vThe heat transferred by the trough 1 creates narrow molten or liquid zones 21 of germanium which are separated by solid zones 22 of germanium. At the frequency employed, 450 kilocycles, even though the heating current penetrates the trough 1 only about two millimeters inward, the liquid zones 21 formed are essentially the same width as the coils 5, 6, and 7 because the slots '19 lead the heat into these narrow zones -21 and diminish thermal conduction along the longitudinal axis lof the trough 1. If the trough 1 isnot slotted in structure, the heat is carried along the length of the trough as well as inw-ard, the liquid zones 21 become wider, and the purification obtained is lower. The slots 19 Ialso allow the solid zones 22 to form more rapidly due to convective cooling which occurs -When the inert gas passes through the slots. Therefore, the slots 19 concentrate the heat in narrow zones which are successively melted as the 'trough 1 moves along the tube 2, -and the slots a-lso act to keep the solid zones relatively cool. It should be noted that the width of the coils 5, 6, and 7 is also kept small; about -four turns per coil have been used successfully for the purposes of this invention. Also, the cross-sectional area of the trough 1 is designed, as previously described, to nearly ll the cross-sectional area of the tu-be 2 in order to bring the trough as close .as possible to the highfrequency coi-ls. Both of these features aid in keeping the liquid zones 21 narrow. Therefore, by narrowing the liquid zones 21 as the semiconductor ma-terial 18 is progressively melted and solidified in the zone-refining apparatus, the degree of purification achieved is greatly increased over that realized by using structures which produce wider liquid zones.

However, -it shou-ld be understood that the invention is not limited to the particular details described above, as many equivalents will suggest themselves to those skilled in the art. For example, the length and diameter of the trough and the distance between the slots are not critica-l but .are determined by the relative sizes of the tube 2 and the coils .3, 4i, 5, 6, '7, and 8. Likewise, the depth of the semiconductor material 1-8 maybe varied but should not be g-reat enough to allow the open top of the material to become too cool. Also, the depth of the slots 19 and the reduced sections 23 may be varied as the crosssectional area and construction 'of the trough `1 are changed. Therefore, it is desired that the appended Yet the diameter of the trough 1 claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A zone-refining apparatus comprising an elongated body vof electrically-conductive material having a recess therein 'adapted to receive the substance to be zone rened, means supplying heat only into a restricted narrow zone in said body, mea-ns for successively applying said tirst means to successive narrow zones in said body whereby narrow zones in said substance may be melted, each of said zones being transverse to the longitudinal -axis of said body and substantially less in length than -said recess, and means inhibiting the -ow yof heat along said longitudinal axis of said body-to a substantially greater degree than through -said transverse axis thereof.

2. A zone-reiining .apparatus .comprising an elongated body `of electrically-conductive material having a recess therein adapted to receive the substance to be zone relined, means supplying heat only into a restricted narrow zone in said body, means for successively applying said tirs-t means to successive narrow zones in said body whereby narrow zones -in said substance may be melted, each of said zones 'being transverse :to the longitudinal axis of said body and substantially less in length than said recess, and Ia plurality of transverse slots spaced along the length of said body whereby said heat is concentrated in each of said narrow zones in said substance.

3. A Zone-refining apparatus comprising an elongated body of electrically-conductive material having a recess therein adapted to yreceive the substance to be zone retined, means supplying heat only into -a restricted narrow zone in said body, means for moving said body at a predetermined rate in a lpredetermined direction to successively apply said first means to successive narrow zones in said body whereby narrow zones in said substance may be melted, ea-ch of said zones being transverse t-o `the longitudinal axis of said body and substantially less in length than said recess, and a plurality of transverse slots spaced along the length of said body to substantially increase and concentra-te the flow yof heat in each of said narrow zones when said body is moving.

4. A zone-refining apparatus comprising 'an elongated body of electrically-conductive material having .a recess therein adapted to receive the subst-ance to be zone re# lined, a set of spaced induction heating coils each lof which is adapted to supply heat only into a restricted narrow zone in said body, means for moving said body at a predetermined rate in a predetermined direction to successively apply said coils to successive narrowzones in said body whereby narrow zones in said substance may be melted, each of said zones -being transverse to` the longitudinal axis of said body and substantially iles-s in length than said recess, and a plurality of transverse slots spaced along the length of said body to substantially i-ncrease the exchange of heat from said coils to eachof said zones when said body is moving.

5. A zone-refining apparatus comprising an elongated body of graph-ite having a recess therein adapted to receive germanium to be zone refined, means supplying' through which an elongated body vmay bernoved, an y elongated body of electrically conductive material having a recess therein adapted to receive the subs-tance ,to-be zone refined and ,located in said chamber,a set yof spaced induction heating coils eachr of which is adapted tosupply heat V only into a restricted narrow zone in said body,

means for moving said body at a predetermined rate in a predetermined direction to successively apply s-aid coils to successive narrow zones in said body whereby narrow zones in said substance may be melted, each of said zones being transverse to a longitudinal axis of said body and substantially less lin length than said recess, means for spacing the end sections of said -body from said chamber, land :means to inhibit the flow of heat from each of said end sections to said spacing means.

References Cited in the le of this patent UNITED STATES PATENTS 2,192,743 Howe Mar. '5, '1940 OTHER REFERENCES Principles of Zone Melting, Bell Telephone System, Technical Publication Monograph 2000, October 1952. 

1. A ZONE-REFINING APPARATUS COMPRISING AN ELONGATED BODY OF ELECTRICALLY-CONDUCTIVE MATERIAL HAVING A RECESS THEREIN ADAPTED TO RECEIVE THE SUBSTANCE TO BE ZONE REFINED, MEANS SUPPLYING HEAT ONLY INTO A RESTRICTED NARROW ZONE IN SAID BODY, MEANS FOR SUCCESSIVELY APPLYING SAID FIRST MEANS TO SUCCESSIVE NARROW ZONE IN SAID BODY WHEREBY NARROW ZONES IN SAID SUBSTANCE MAY BE MELTED, EACH OF SAID ZONES BEING TRANSVERSE TO THE LONGITUDINAL AXIS OF SAID BODY AND SUBSTANTIALLY LESS IN LENGTH THAN SAID RECESS, AND MEANS INHIBITING THE FLOW OF HEAT ALONG SAID LONGITUDINAL AXIS OF SAID BODY TO A SUBSTANTIALLY GREATER DEGREE THAN THROUGH SAID TRANSVERSE AXIS THEREOF. 